Novel EPDM rocket motor insulation

ABSTRACT

A novel and improved EPDM formulation for a solid propellant rocket motor is described wherein hexadiene EPDM monomer components are replaced by alkylidene norbornene components, and, with appropriate adjustment of curing and other additives, functionally required rheological and physical characteristics are achieved with the desired compatibility with any one of a plurality of solid filler materials, e.g., powder silica, carbon fibers or aramid fibers, and with appropriate adhesion and extended storage or shelf-life characteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/310,005, filed Dec. 3, 2002, pending, which is a divisional of application Ser. No. 09/481,709 filed Jan. 12, 2000, now U.S. Pat. No. 6,566,420, issued May 20, 2003.

This application claims the benefit of priority of Provisional Application Nos. 60/115,855, 60/115,856, 60/115,859, and 60/115,857, each filed on Jan. 13, 1999.

GOVERNMENT LICENSING CLAUSE

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract NAS 8-38100 awarded by the National Aeronautics and Space Administration (NASA) and contract N00030-97-C-0100 awarded by the U.S. Department of the Navy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a novel composition and method for providing insulation for solid propellant rocket motors and, more particularly, to EPDM compositions having fibrous components such as carbon fibers or powder fillers such as silica, or also containing KEVLAR® reinforcing fibers and suitable for internal and external insulation applications on such rocket motors.

2. State of the Art

It is generally accepted current industry practice to prepare insulations for solid propellant rocket motors from a polymeric base importantly composed of an EPDM (ethylene-propylene-diene monomer) terpolymer blend and containing 1,4-hexadiene (HD) as one of the diene monomer components of the EPDM blend.

This EPDM terpolymer, which is commonly designated as the primary EPDM terpolymer since it is present in a higher concentration than the secondary EPDM terpolymer, has been established as a standard for solid propellant rocket motor insulations due to its superior ablation characteristics, excellent physical properties and processability.

For instance, an exemplary carbon fiber-filled rocket motor insulation composed of NORDEL® 1040 as the primary terpolymer is commonly known in the industry as the STW4-2868 thermal insulation and has the following composition as shown in Table 1: TABLE 1 STW4-2868 THERMAL INSULATION FORMULATION (carbon fiber; parts by weight) Parts by Ingredient Function Weight NORDEL ® 1040 Primary EPDM terpolymer base 80 NEOPRENE ® FB Secondary polymer base 20 Zinc oxide Activator 5 Sulfur Curative 1 HAF carbon black Pigment 1 MBT Accelerator 1 AGERITE ® Resin D Antioxidant 2 AGERITE ® HPS Antioxidant 1 Tellurac Accelerator 0.50 SULFADS ® Accelerator 0.75 VCM carbon fibers Filler 41 Total Parts by Weight 153.25

Alternatively, solid rocket motor insulations are also composed of compositions employing finely divided powder silica as a filler, with or without the added presence of a fibrous reinforcing agent.

Exemplary silica-filled rocket motor insulations have also included NORDEL® 1040 and NORDEL® 2522 as the primary terpolymer in their formulations and the resulting compositions are respectively commonly known in the industry as the 053A and DL1375 thermal insulations. They have the following compositions shown in Table 2: TABLE 2 THERMAL INSULATION FORMULATION (silica filled; parts by weight) DL1375 053A (parts by (parts by Ingredient Function weight) weight) NORDEL ® 1040 Primary EPDM 80 terpolymer base NORDEL ® 2522 Primary EPDM 80 terpolymer base NEOPRENE ® FB Secondary polymer base 20 20 Zinc oxide Activator 5 5 Sulfur Curative 1 1 AGERITE ® Resin D Antioxidant 2 2 AGERITE ® HPS Antioxidant 1 1 Captax Accelerator 1 1 Tellurac Accelerator 0.5 0.5 SULFADS ® Accelerator 0.75 0.75 HISIL ® 233 Filler 35.5 35.5 Total Parts by Weight 146.75 146.75

In addition, an EPDM terpolymer comprising the HD monomer is sold under the tradename NORDEL® 2722E. An exemplary silica-filled rocket motor insulation comprising NORDEL® 2722E as the secondary terpolymer is commonly known in the industry as the DL1552A thermal insulation and has the following composition as shown in Table 3: TABLE 3 DL1552A THERMAL INSULATION FORMULATION WITH SILICA Parts by Ingredient Function Weight BUNA ® EP T 3950 (Bayer Corp., Fiber, Additives and Primary EPDM 75 Rubber Division of Orange, Texas) terpolymer base NORDEL ® 2722E (DuPont Dow Elastomers) Secondary EPDM 20 terpolymer base with high ethylene content WINGTACK ® 95 (hydrocarbon resin) (Goodyear Tire and Tackifier 7 Rubber Co., Chemical Division of Beaumont, Texas) IRGANOX ® 1010 (tetrakis[methylene-3-(3′5′-di-tert-butyl- Antioxidant 1 4′-hydroxyphenyl) propionate]methane) (Ciba Specialty Chemicals, Additives Division, Tarrytown, N.Y.) TRYCOL ® DA-6 (decyl polyoxyethylene alcohol) Wetting agent 0.5 (Chemical Associates, Inc. of Copley, Ohio) Stearic acid (including palmitic acid) (Harwick Standard Cure activator 1 Distribution Corp. of Akron, Ohio) HISIL ® 233 (silica hydrate) (PPG Industries, Inc. of Lake Reinforcing filler 45 Charles, Louisiana) Aluminum oxide C (Al₂O₃) (Degussa Corporation of Reinforcing filler 0.3 Ridgefield Park, N.J.) N330 carbon black (Columbian Chemicals Co. of Marietta, Pigment and 1 Ga.) reinforcing filler KALENE ® 1300 (butyl gum elastomer) (Hardman Division Co-vulcanizing 20 of Harcros Chemicals, Inc. of Belleville N.J.) plasticizer HYPALON ® 20 (chlorosulfonated polyethylene) (DuPont Cure activator 5 Dow Elastomers) AGERITE ® Resin D (polymerized trimethyl Antioxidant 0.25 dihydroquinone) (R.T. Vanderbilt Co., Inc. of Buena Park, Ca.) TZFD-88p (zinc oxide dispersed in an EPDM binder) Cure activator 2 (Rhein Chemie Corp. of Trenton, N.J.) SP 1056 (bromomethyl alkylated phenolic resin) Curing agent 15 (Schenectady Int'l, Inc. of Schenectady, N.Y.) Total Parts by Weight 193.05

An exemplary aramid fiber-filled rocket motor insulation comprising NORDEL® 1040 is commonly known in the industry as R196 thermal insulation and has the following composition as shown in Table 4: TABLE 4 R196 THERMAL INSULATION FORMULATION WITH KEVLAR ® Parts by Ingredient Function Weight NORDEL ® 1040 (EPDM terpolymer) Polymer base 80 NATSYN ® 2200 (polyisoprene) (Goodyear Tire and Polymer base 20 Rubber Co., Chemical Division of Akron, Ohio) WINSTAY ® S (syrenated phenols) (Goodyear Tire and Antioxidant 1.0 Rubber Co., Chemical Division of Akron, Ohio) Dechlorane Plus 515 (1,2,3,4,7,8,9,10,13,13,14,14- Flame retardant 40 dodecachloro-1,4,4a,5,6,6q,7,10,10a,11,12,12a- dodecahydro-1,4,7,10-dimethanodibenzo (a,e) cyclooctene) (Occidental Chemical Corporation of Dallas, Texas) Antimony oxide (Sb₂O₃) (Harcros Chemicals, Inc. of Flame retardant/filler 20 Kansas City, Kansas) ¼″ KEVLAR ® fiber (aramid staple fiber) (E.I. duPont Fiber 20 de Nemours and Co., of Wilmington, Delaware) VAROX ® DBPH-50 (2,5-dimethyl-2,5-di(t-butylperoxy) Curing agent 2.5 hexane on a carrier) (R.T. Vanderbilt Co., Inc. of Buena Park, Ca.) Total Parts by Weight 183.5

Numerous past efforts to develop effective replacements for these standard solid rocket motor insulation formulations have not been successful.

The only manufacturer currently producing the foregoing primary EPDM terpolymer in adequate quantities to meet the demands of the rocket motor insulation industry is DuPont Dow Elastomers of Beaumont, Tex., which markets and sells an EPDM terpolymer comprising the HD monomer under the tradename NORDEL® 1040 and NORDEL® 2522.

However, the ability of the industry to produce STW4-2968, DL1375, 053A, DL1552A, R196 and other thermal insulations containing NORDEL® 1040 and NORDEL® 2522, and NORDEL® 2722E as a primary or secondary EPDM terpolymer has recently been placed in jeopardy due to the announcement by DuPont of its intention to cease production of NORDEL® 1040, 2522, 2722E and, generally, other EPDM polymers formed from 1,4-hexadiene. There is, therefore, a need in this industry, previously not satisfied, to find an effective alternate or a replacement for the above-described standard STW4-2868, DL1375, 053A DL1552A and R196 thermal insulations. Development and formulation of a suitable primary EPDM terpolymer replacement is especially critical for these discontinued NORDEL® insulation formulations.

The requirements for an acceptable, functionally effective, insulation for solid propellant rocket motors are well known to be quite severe due to the extreme conditions to which the insulation is exposed. These conditions not only include exceedingly high temperatures but also severe ablative effects from the hot particles (as well as gases) that traverse and exit the rocket motor interior. Unless the insulation will withstand such conditions, catastrophic failure has and may occur.

U.S. Pat. No. 3,347,047, an early patent describing asbestos fiber-filled insulations, states that flame temperatures encountered in the combustion of propellants, particularly when used as the source of propulsion, necessitating the confinement of the gases of combustion and ultimate release thereof through orifices, are usually accompanied by extremely turbulent flow conditions. All of these features place considerable stress and strain upon the member defining the escape passageway. While the combustion of the propellant in the case of rockets and the like will usually be of short duration, the temperatures and turbulence encountered have been found to very easily destroy even the strongest and most exotic alloys formed of iron, steel, titanium, magnesium, silicon, chromium, beryllium and the like. As a consequence, the projectile structure fails, leading to total destruction thereof through explosion, or in the event that only the exit passageway is destroyed, the projectile proceeds in an erratic, uncontrollable path since its trajectory or path is, at least in part, dependent upon the contour of the passageway through which the gaseous products of combustion pass. That statement still remains fully applicable today.

Therefore, any replacement insulation should exhibit at least comparable temperature-resistant and ablation characteristics and theological and physical properties (e.g., Mooney viscosity) at least equivalent to that of STW4-2868, DL1375, 053A, DL1552A and R196, yet should not otherwise significantly alter the formulation techniques employed for the production of such rocket motor thermal insulation. Additionally, due to the large and growing quantities of solid propellant rocket motor insulation required by the industry, any such replacement EPDM terpolymer candidate should be abundantly available now and into the foreseeable future.

In addition, any replacement EPDM or like terpolymer should satisfy a number of other requirements including wettability of and bond strength with such diverse filler additives as a carbon fiber, aramid fiber, and a silica powder. It is also necessary that such additives be substantially homogeneously dispersed throughout the insulation composition as it is being produced. While standard mixing devices can be employed in the practice of this invention, such as a Banbury mixer, it is a common experience that substantially homogeneous distribution of fibrous additives is not achieved, or achieved only with difficulty, with many elastomeric compositions. Difficulties have been described as in, for instance, during mixing of the components; it can be observed that premature vulcanization may occur as well as other problems that may impede or entirely frustrate effective distribution of the various additives which are essential to the ultimate production of the insulation.

Further, once formulated, the elastomeric composition must also possess acceptable shelf life characteristics such that it remains sufficiently pliable, without becoming fully cured, until used in application to the rocket motor casing. This requirement is essential because the production of a given lot of insulation may have to wait in storage for a number of months prior to use. Typically, the insulation may be stored in large rolls in an uncured or, at most, a partially cured state until ready for use. A number of curing agents are well known and are conventionally employed but still must be compatible with the overall EPDM formulation to permit satisfactory shelf life. This in turn requires a balancing of curing agent activity.

In addition, the formulated insulation should be substantially odorless for obvious reasons and this can require special adjustment of the curing agent components.

After application to the interior (or, if desired, the exterior) of the rocket motor casing and subsequent curing thereof, an acceptable insulation must also exhibit satisfactory bonding characteristics to a variety of adjacent surfaces. Such surfaces include the internal surface of the rocket motor casing itself and the insulation must also exhibit adequate bonding characteristics between itself and the propellant grain, typically with an intermediate liner surface. In turn, the propellant grain in a solid propellant rocket motor is composed of a variety of materials notably including still another elastomer, various combustible materials, and such additional components as aluminum particles.

A functionally acceptable solid propellant rocket motor insulation must meet those requirements and must also survive aging tests. Such rocket motors may be fully fabricated even many months before actual firing and, for tactical weapons especially, sometimes even more than a year or even a plurality of years. For instance, strategic missiles may be stored in silos or submarine launch tubes for decades. Over that period of time, the insulation must continue to remain fully functional without unacceptable migration of its components to or from adjacent interfacial surfaces and adequately retain its elastomeric characteristics to prevent brittleness. This requirement also needs to be satisfied under wide temperature variations. The vibration and physical stress placed on a rocket motor at the time of launch, whether a ground launch or an air firing, is exceedingly high, and brittleness and cracking in the insulation is effectively intolerable, whether from premature or gradual overcure or whatever cause. Even at the end of the burn of the propellant grain within the rocket motor casing, the insulation must remain substantially and functionally intact to avoid potentially catastrophic failures of the entire launch vehicle.

In turn, this means that the insulation composition must meet the ablation limits for protection of the casing throughout the propellant burn without adding undue weight to the motor.

A number of past patents have been granted proposing various solutions to the insulation formulation problem. These include U.S. Pat. No. 3,421,970 (generically describing elastomeric formulations with asbestos); U.S. Pat. No. 3,562,304 (generically describing an elastomeric formulation with asbestos fibers); U.S. Pat. No. 3,637,576 (describing an EPDM formulation with a norbornene component with asbestos fibers); U.S. Pat. No. 4,492,779 (generically describing elastomeric formulations with KEVLAR® fibers); U.S. Pat. No. 4,514,541 (generically a du Pont “master batch” formulation with KEVLAR® fibers, but not an insulation); U.S. Pat. No. 4,550,130 (generically describing a moldable carboxylic acid modified EPDM to enhance affinity to various fillers); U.S. Pat. No. 4,878,431 (generically describing an elastomeric formulation using the EPDM NORDEL® 1040, with KEVLAR® fibers); U.S. Pat. No. 5,364,905 (describing a technique for the in situ polycondensation formation of aramid fibers, but not referring to rocket motor insulations); U.S. Pat. No. 5,498,649 (describing a polyamide/maleic anhydride modified EPDM with KEVLAR® fibers for a rocket motor insulation); U.S. Pat. No. 5,821,284 (a KEVLAR® fiber-filled insulation containing an EPDM illustrated by NORDEL® 2522 in combination with ammonium salts); and U.S. Pat. No. 5,830,384 (generically referring to EPDM's with a “dry water” silica additive for cooling purposes). None of these patents address nor effectively solve the problem faced by the present invention. In fact, the frequent reference to NORDEL® 1040 or NORDEL® 2522 serves to confirm the observation that these particular elastomers are well-nigh the standard in the rocket motor insulation industry.

Accordingly, the search for a functionally satisfactory elastomeric insulation composition requires discovery and implementation of an extraordinarily complex combination of characteristics. The criticality of the material selection is further demonstrated by the severity and magnitude of the risk of failure. Most insulations are of necessity “man-rated” in the sense that a catastrophic failure can result in the loss of human life—whether the rocket motor is used as a booster for launch of the space shuttle or is carried tactically underneath the wing of an attack aircraft. The monetary cost of failure in satellite launches is well-publicized and can run into the hundreds of millions of dollars.

One well-known potential point of failure is the appearance of voids or cracks in the insulation which could lead to the penetration of the rocket motor casing itself. The resultant dispersion of hot gases may not only lead to destruction of the motor generally but can at least lead to its being thrown off its intended course or trajectory with several unhappy results. In such events, the vehicle itself will either self-destruct or be intentionally destroyed, or the satellite will be launched into a useless orbit.

Therefore, one of the most difficult tasks in the solid propellant rocket motor industry is the development of a suitable, acceptable insulation composition that will meet and pass a large number of test criteria to lead to its acceptability.

Furthermore, any replacement EPDM terpolymers should not be susceptible to obsolescence issues nor discontinuance in future supply thereof.

BRIEF SUMMARY OF THE INVENTION

The present invention addresses a crucial long standing need in the industry for an acceptable substitute for the STW4-2868, DL1375, 053A, DL1552A and R196 insulations by providing a reformulated rocket motor thermal insulation notably comprising a suitable primary or secondary terpolymer replacement for the 1,4-hexadiene-based EPDM and one that minimizes the degree of modification to the existing formulation methods and also as to the ultimate functional properties of the STW4-2868, DL1375, DL155A, 053A and R196 thermal insulations.

The present invention relates to a rocket motor insulation formulation comprising, as a primary or secondary terpolymeric base, an EPDM terpolymer formed from at least one alkylidene norbornene, especially ethylidene norbornene (ENB) as the diene component.

Exemplary EPDM terpolymers that may be used according to this invention comprise those having an alkylidene diene, particularly an ENB diene, component include KELTAN® 4506, KELTAN® 1446A, KELTAN® 2308, NORDEL® IP NDR520, and NORDEL® IP NDR-640, each of which may be substituted into the STW4-868, DL1375, R196 and 053A insulation for the NORDEL® 1040 without requiring significant modifications to the standard STW4-2868, DL1375, 053A, DL1552A and R196 thermal insulation formulation methods or to the resulting multitude of functionally acceptable properties. Other exemplary terpolymers including high-thylene-content EPDM terpolymers formed from an ENB diene component are NORDEL® IP NDR-3722p and BUNA® EP T 2370, which may be substituted into the DL1552A for the NORDEL® IP NDR-2722E without requiring significant modifications to the DL1552A formulation. NORDEL® IP NDR-3725 has also been used but the supplier (du Pont) has indicated that due to low demand, it now prefers a different formulation, NORDEL® IP NDR-3722, with a lower diene content of about 0.5% versus about 2.5% for NORDEL® IP NDR-3725p.

It has now been found that only a small proportion of the ENB diene component is sufficient for incorporation in such elastomers, say from about 2 to about 10 wt. %, preferably from about 2 to about 7 wt. %, and with the balance of the olefin content of the composition composed of ethylene and propylene, with the ethylene forming from about 40 to about 80 wt. %, preferably from about 50 to about 75 wt. %, and with the remainder being propylene. Trace amounts of other dienes may also be present to induce branching in the elastomer. Generally, the only significant modification that is required involves the selection of a less-reactive curing agent to offset the higher reactivity (relative to HD) of ethylidene norbornene (ENB). Furthermore, NORDEL® IP NDR-3722 and BUNA® EP T 2370 are not presently foreseen as being susceptible to obsolescence issues.

Other aspects and advantages of the invention will be apparent to those skilled in the art upon reading the specification and appended claims which, when read in conjunction with the accompanying drawings, explain the principles of this invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings serve to elucidate the principles of this invention. In such drawings:

FIGS. 1 a-1 g are schematic cross-sectional views of a rocket motor depicting various regions in which the insulation of this invention may be applied;

FIG. 2 is a schematic of a char motor suitable for conducting the ablation tests reported herein; and

FIG. 3 is a cut-away view of a solid propellant rocket motor illustrating the outer case, the internal insulation layer, the propellant grain, and the insulation of the ignitor closure structure.

DETAILED DESCRIPTION OF THE INVENTION

As shown schematically in FIGS. 1 a-3, the manner in which the inventive insulation may be applied to various parts of a rocket assembly is illustrated. The rocket assembly includes, but is not limited to, a system tunnel floor plate shear ply 10, a LSC blast shield 12, a stiffener stub hole plug 14, a stiffener stub insulation 16, an external joint weather seal 18, a T-ring insulation 20, an aft dome internal insulation 22, and a center segment aft end internal insulation 24 (underneath the flap).

In FIG. 2, the schematic cross-section of a char motor test assembly is illustrated wherein the propellant is contained in the beaker 30, with low-velocity insulation test specimens located at region 32 upstream of the throat 34, with medium-velocity test specimens located in region 36 and with high-velocity insulation test specimens located in the region 38. Generally, such a char test motor assembly permits the location of a plurality of different insulation formulation test specimens about the circumference at any of regions 32, 36 or 38 in the conventional manner.

FIG. 3 is a cut-away schematic view of a typical rocket motor illustrating the case 40, the nozzle 42, the propellant grain 44 with its center bore 46 and the internal insulation layer shown at 48. The insulation of the ignitor closure structure is indicated at 50.

In accordance with one embodiment of this invention, the inventive rocket motor insulation formulation comprises, as a primary polymeric base, an EPDM terpolymer in which the diene component of the EPDM is composed of at least one alkylidene norbornene and, in particular, ethylidene norbornene (ENB).

The selected EPDM terpolymer should be substitutable into the STW4-2868, DL1375, 053A, DL1552A and R196 thermal insulation formulation (Tables 1 and 2) without requiring significant modification of the present techniques employed for the formulation thereof. It is a further highly desirable feature of the present invention that the insulation formulation may be composed of readily available commercial materials, provided that such compositions are properly assembled and blended together for the final insulation material.

Suitable EPDM terpolymers having an ENB diene component for use in this invention include, without limitation, and as stated above, KELTAN® 4506, KELTAN® 1446A, KELTAN® 2308, each of which is available from DSM of the Netherlands, and NORDEL® IP 4520 and NORDEL IP 4640, both of which are and continue to be available from DuPont Dow Elastomers.

These materials have the following respective contents as derived from the manufacturer's data literature: ENB Content Ethylene Content Mooney Wt % Wt % Viscosity KELTAN ® 4506 4.5 54 40 KELTAN ® 1446A 7 59 10 KELTAN ® 2308 2 74 24 NORDEL ® IP 4520 5 51 20 NORDEL ® IP 4640 5 55 40 The remaining content is propylene with traces of certain dienes used to produce branching in the molecular structure.

In accordance with another preferred embodiment, the secondary EPDM terpolymer is/are NORDEL® IP NDR-3722 and/or BUNA® EP T 2370, which include ENB as their diene monomers and are respectively available from DuPont Dow and Bayer Corporation and manufactured in relatively large capacities.

An exemplary formulation is set forth in Table 5: TABLE 5 Ingredient Parts by Weight Primary EPDM terpolymer 70-80 Secondary EPDM terpolymer 15-25 with ENB diene monomer Tackifier  5-10 Antioxidant 1-3 Wetting agent 0-1 Curing activator  5-10 Silica filler 40-50 Pigment 0-3 Plasticizer 15-25 Curing agent 10-20

The primary EPDM preferably has a sufficiently high diene content to provide a more reactive polymer to decrease cure time. Preferably, the alkylidene diene content of the primary EPDM is in a range of about 2-12 wt %, more preferably about 10-12 wt %. Additionally, the primary EPDM terpolymer preferably has a medium ethylene content of from about 56 wt % to about 65 wt %.

In a more preferred embodiment, the primary and secondary terpolymer components, tackifier, antioxidant, wetting agent, curing activator, filler, pigment, plasticizer, and silica are the same as those set forth in Table 3 above and are present in the concentrations specified in Table 3. However, when following that formulation composition on simply a “drop in” basis, it was observed that an unsatisfactory scorch characteristic developed (e.g., a scorch time of about 5.9 minutes as against an 8 minute minimum time allowed per specification). Similarly, substituting BUNA® EP T 2370 in place of NORDEL® 2722E led to similar results of an unsatisfactory scorch time of 6.6 minutes and a high Mooney viscosity of 119. Consequently, according to this invention, a less-reactive brominated phenolic resin curing agent, for instance, having a lower reactivity than that used in the DL1552A formulation (i.e., SP 1056 containing about 6 wt % bromine) was required and was selected in order to compensate for the observed higher reactivity of ENB. This then led to acceptable scorch characteristics, and the use of cure temperatures of about 320° F. provided similar cure rates as were used for DL1552A. The resulting physical properties were also acceptable. Preferably, the resin curing agent of the reformulated insulation formulation is SP 1055 (manufactured by Schenectady International), which contains about 3% bromine by weight.

It will be observed that these materials may have significant variations in specific norbornene content, and also in the ethylene/propylene content ratio as well as in the Mooney viscosity, yet each has been found to be effective if selected for use in production of a rocket motor insulation.

Adhesion-promoting secondary polymers that may also be used in the formulation include elastomer modifiers, especially polar polymers. Among such suitable secondary polymers are chlorosulfonated polyethylene, such as HYPALON® 20 from DuPont Dow, and polychloroprene. Polychloroprene polymers are available from DuPont Dow under the tradenames NEOPRENE® FB, NEOPRENE® TW, and NEOPRENE® GRT.

An exemplary plasticizer for the inventive formulation is the EPDM-based TRILENE® 67A (Uniroyal).

Tackifiers may also optionally be used. An example of a suitable tackifier is AKROCHEM®θ P-133.

One or more antioxidants are also preferably included within the inventive insulation formulation. Preferred antioxidants include polymerized 1,2-dihydro-2,2,4-trimethylquinoline (AGERITE® Resin D) and mixed octylated diphenylamines (AGERITE® Stalite S), each of which is available from R.T. Vanderbilt Co.

Various powder silica fillers are well known in other elastomeric combinations and may be used, including HISIL® 233.

An exemplary carbon fiber is VMC carbon fiber.

Suitable cure activators may include metal oxides, such as zinc oxide and magnesium oxide (e.g., ELASTOMAG® 170, from Morton Chemical Co.).

The curing package preferably includes at least one phosphate cure accelerator, including, by way of example, RHENOCURE® AP-5, RHENOCURE® AP-7, RHENOCURE® AP-3, RHENOCURE® ZADT/G, and RHENOCURE® S/G, which are available from Rhein Chemie and Accelerator VS, available from Akro Chem. Additional cure accelerators that may be used in combination with the phosphate cure accelerator include butyl zimate, ALTAX®, AKROFORM®, Delta P. M., SULFADS®. While the use of Accelerator VS was initially unacceptable because of the foul odor problem it generated, it has also now been found that such formulations can be prepared with no significant odor when about 1.0 phr magnesium oxide is added thereto.

Sulfur curing agents are preferred for the formulation. A suitable sulfur-curing agent is AKROSPERSE® IS-70 from Akro Chem. Elemental sulfur can also be used.

Batches of insulation containing silica powder generally may be formulated in an internal mixer with the following two-pass mix procedure. In the first pass, the mixer speed may be set to approximately 40 rpm and all of the components other than the curing agents and accelerators are added to the internal mixer. (The silica was added last in the first pass.) The mixing may be performed at a temperature of about 300° F. Suitable mixing times depend on the temperature and mixing speed, and are ascertainable to the skilled artisan without an undue amount of experimentation.

In the second pass, the mixer speed was set to about 40 rpm. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added, then the remaining half of the master batch was added to the mixer. The second mixing step was performed at a temperature of approximately 180° F. to 190° F. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature prior to evaluating its rheological and physical properties.

The shaping and curing of the inventive insulation may be performed in accordance with techniques known in the art.

Exemplary new formulations containing a silica powder filler are set forth in Table 6 below with concentrations shown by weight. TABLE 6 EXAMPLES OF SILICA-FILLED EPDM INSULATION FORMULATIONS Ingredient RDL5338 RDL5342 RDL5343 RDL5347 RDL5363 RDL5255 RDL5335 RDL5201 RDL5220 KELTAN ® 4506 70.0 73.0 73.0 55.0 55.0 KELTAN ® 1446A 20.0 17.0 17.0 KELTAN ® 2308 35.0 35.0 NORDEL ® IP 4520 80.0 80.0 NORDEL ® IP 4640 80.0 80.0 HYPALON ® 20 10.0 10.0 10.0 10.0 10.0 NEOPRENE ® FB 20.0 20.0 NEOPRENE ® TW 20.0 20.0 NEOPRENE ® GRT TRILENE ® 67A KRATON ® L-2203 AKROCHEM ® P-133 5.0 5.0 5.0 5.0 5.0 5.0 5.0 AGERITE ® Resin D 2.0 2.0 2.0 2.0 AGERITE ® Stalite S 2.0 2.0 2.0 2.0 2.0 HISIL ® 233 35.5 35.5 35.5 35.5 35.5 35.5 35.5 37.0 37.0 ELASTOMAG ® 170 0.5 0.5 Zinc Oxide 3.0 3.0 3.0 3.0 3.0 4.0 4.0 4.0 4.0 Butyl Zimate 0.50 0.50 0.50 RHENOCURE ® AP-5 3.50 3.50 3.50 RHENOCURE ® AP-7 RHENOCURE ® AP-3 2.90 3.85 RHENOCURE ® ZADT/G 0.80 1.00 RHENOCURE ® S/G 0.60 0.80 ALTAX ® 1.10 1.10 1.20 1.10 AKROFORM ® Delta P.M. 0.45 0.20 SULFADS ® 0.50 Accelerator VS 2.70 2.70 AKROSPERSE ® IS-70 1.10 1.10 1.20 1.20 1.40 1.00 Sulfur 0.35 0.45 0.40 EXAMPLES OF SILICA-FILLED EPDM FORMULATIONS Ingredient RDL5370 RDL5380 RDL5381 RDL5194 RDL5222 RDL5273A RDL5298 RDL5277 RDL5279 KELTAN ® 4506 KELTAN ® 1446A KELTAN ® 2308 NORDEL ® IP 4520 80.0 80.0 80.0 90.0 90.0 90.0 80.0 NORDEL ® IP 4640 90.0 90.0 HYPALON ® 20 10.0 10.0 10.0 10.0 NEOPRENE ® FB NEOPRENE ® TW NEOPRENE ® GRT 20.0 20.0 20.0 TRILENE ® 67A 10.0 KRATON ® L-2203 10.0 10.0 AKROCHEM ® P-133 5.0 5.0 5.0 5.0 5.0 5.0 5.0 9.0 9.0 AGERITE ® Resin D 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 AGERITE ® Stalite S HISIL ® 233 37.0 37.0 37.0 35.5 35.5 35.5 36.5 38.0 38.0 ELASTOMAG ® 170 0.50 0.50 0.50 Zinc Oxide 4.00 4.00 4.00 3.00 3.00 4.00 3.00 3.00 3.00 Butyl Zimate 0.50 0.40 0.15 RHENOCURE ® AP-5 3.40 3.15 4.45 RHENOCURE ® AP-7 3.00 RHENOCURE ® AP-3 3.85 3.85 RHENOCURE ® ZADT/G 1.00 1.00 0.90 RHENOCURE ® S/G 0.80 0.80 ALTAX ® 1.00 1.00 1.20 AKROFORM ® Delta P.M. 0.25 SULFADS ® 0.82 Accelerator VS 2.70 2.70 AKROSPERSE ® IS-70 0.95 1.10 1.34 1.25 1.40 Sulfur 0.40 0.50 0.50 0.40 Ingredient RDL5319 RDL5320A RDL5350 RDL5351A RDL5367 RDL5185 RDL5186 RDL5205 RDL5276 KELTAN ® 4506 KELTAN ® 1446A KELTAN ® 2308 NORDEL ® IP 4520 40.0 40.0 45.0 45.0 45.0 NORDEL ® IP 4640 50.0 50.0 45.0 45.0 45.0 80.0 80.0 80.0 80.0 HYPALON ® 20 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 NEOPRENE ® FB NEOPRENE ® TW NEOPRENE ® GRT TRILENE ® 67A 10.0 10.0 10.0 10.0 KRATON ® L-2203 AKROCHEM ® P-133 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 AGERITE ® Resin D 2.0 2.0 2.0 2.0 AGERITE ® Stalite S 2.0 2.0 2.0 2.0 2.0 HISIL ® 233 35.5 35.5 35.5 35.5 35.5 35.5 35.5 35.5 35.5 ELASTOMAG ® 170 Zinc Oxide 3.0 4.0 3.0 4.0 3.0 3.0 3.0 3.0 3.0 Butyl Zimate 0.40 0.40 0.40 RHENOCURE ® AP-5 3.15 3.15 3.50 RHENOCURE ® AP-7 3.00 RHENOCURE ® AP-3 3.85 3.85 RHENOCURE ® ZADT/G 0.80 0.80 1.00 RHENOCURE ® S/G 0.80 0.80 ALTAX ® 1.00 1.00 1.10 AKROFORM ® Delta P.M. 0.35 SULFADS ® 0.50 Accelerator VS 2.70 2.70 AKROSPERSE ® IS-70 1.00 1.00 1.00 1.00 1.00 1.05 Sulfur 0.40 0.80 0.40

In addition, individual batches of insulation formulations containing silica filler were prepared with NORDEL® IP NDR-3725 (Example 1, Table 8 below), BUNA® EP T 2370 (Example 2), and NORDEL® 2722E (Comparative Example) as the secondary EPDM terpolymers having high ethylene contents. However, in these formulations it has been observed that there is an increased reactivity of the ethylidene norbornene monomers as used in Examples 1 and 2 (compared to the 1,4-hexadiene monomer of the Comparative Example A) and, accordingly, SP 1055 (3 wt % bromine) was used in Examples 1 and 2 and SP 1056 (6 wt % bromine) was used in the Comparative Example A (Table 8) in equal quantities for comparison purposes. In addition, another formulation, RDL5654 was also prepared in much the same way, but substituting NORDEL® IP NDR-3722p for NORDEL® IP NDR-2722E, and SP 1055 was used in place of SP 1056. TABLE 7 Diene Ethylene Propylene content Polymer content % content % % type Viscosity NORDEL ® IP 71 26.5 2.5/ENB 25 NDR-3722 BUNA ® EP T 2370 71 26   3/ENB 16 NORDEL ® 2722E 72 22 6.4/HD 26

The batches were each formulated in a laboratory scale Reliable Rubber & Plastics Machinery Company Model R-260 internal mixer having a net chamber volume of 4260 cubic centimeters. A 3000 gram batch was prepared by a two-pass mix procedure.

In the first pass, the mixer speed was set at 40 rpm, and the primary and secondary polymer components, tackifier, and antidegradant were added to the mixing chamber in the concentrations set forth in Table 3 and masticated for one minute. Subsequently, the remaining ingredients were added (same concentrations as in Table 3), and the mixer speed was increased to 60 rpm. The batches were removed from the internal mixer after mixing for a total of approximately 7 minutes. The temperature at the time of removing the batch was 300° F. to 320° F. The master batch was sheeted out on a 6″×13″ Farrel Corporation laboratory two-roll mill and allowed to cool to room temperature.

In the second pass, the mixer speed was set to 40 rpm. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added (same concentrations as set forth in Table 3), then the remaining half of the master batch was added to the mixer. After a total mix time of approximately 40 seconds, the final mix was removed from the mixer. The actual temperature at the time of removal was approximately 200° F. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature prior to evaluating its Theological and physical properties.

The rheological properties of these several formulations are reported in Table 8: TABLE 8 RHEOLOGICAL PROPERTIES (silica filler) Example 1 Example 2 Comparative Property [RDL5294] [RDL5331] RDL5654 Example A (1) Mooney viscosity (ML 1 + 4 at 63.4 61.0 65.4 68.8 212° F.) (ASTM D 1646) (2) Mooney scorch (MS + 1 at 250° F., 21.5 18.1 27.1 11.8 min.) (ASTM D 1646) (3) Oscillating disk rheometer (ODR — at 320° F., 5° arc) (ASTM D 2084) properties: (a) ML (minimum torque, in.-lb.) 17.8 16.4 17.1 21.4 (ASTM D 2084) (b) MH (maximum torque, at 2 hrs, 91.0 95.0 86.8 97.7 in.-lb.) (ASTM D 2084) (c) Ts2 (time to 2 point rise above 2.3 1.8 2.4 1.5 minimum viscosity, in min.) (ASTM D 2084) (d) MC(90) (in.-lb.) 83.7 87.1 79.8 90.1 (ASTM D 2084) (e) Tc (90) (min) (ASTM E 2084) 74.0 73.5 77.1 75.5

The physical characteristics of these formulations are reported as follows in Table 9: TABLE 9 PHYSICAL PROPERTIES Specification Example 1 Example 2 Comparative Properties Requirement [RDL5294] [RDL5331] RDL5654 Example A Shore A hardness 69-79 73.8 74.2 72.4 71.8 (ASTM D 2240) Ash content (%) 20-26 22.6 22.4 22.2 22.7 (ASTM D 297) Specific gravity 1.04-1.07 1.051 1.058 1.056 1.054 (ASTM D 792) Tensile strength, perpendicular 1450 min.  2420 2480 2210 2280 (psi ASTM D 412) Elongation parallel 450 min. 703 693 643 636 (%; ASTM D 412) 100% modulus (psi) — 347 370 377 371 Tear resistance (pli) 170 min. 233 238 230 221 (ASTM D 624) Specific heat (BTU (lb.)⁻¹ (° F.)⁻¹ — 0.445 0.442 — 0.443 (ASTM E 1269) Thermal conductivity (BTU — 0.134 0.133 — 0.129 (ft)⁻¹(in.)⁻¹(° F.)⁻¹ (ASTM E 1225)

Measurements were also made of the resulting ablation rates for these formulations. TABLE 10 MATERIAL ABLATION RATE (MILS/SECOND) Comparative Example 1 Example 2 Example A Low Mach Region (0.0028) 3.00 3.13 3.40 Medium Mach Region 7.34 7.94 7.60 (0.0318-0.0386) High Mach Region 19.60 17.37 20.11 (0.0397-0.0634)

The ablation tests for Table 10 were performed as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the three velocity regions of the motor (low, medium, and high Mach) were net molded using conventional compression molding techniques. The low Mach region had space for 15 specimens, while the medium and high Mach regions had space for 4 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 11.7 seconds and 936 psi, respectively.

The cure characteristics and processability of the inventive Examples closely matched those of the Comparative Example A. The cure rates, as indicated by the 90 percent cure time tc(90), of Examples 1 and 2 were equivalent to that of the Comparative Example A. Similarly, the physical properties and ablative performance of the materials prepared in accordance with Examples 1 and 2 were substantially equivalent to those of the Comparative Example and were well within the specification requirements. Thus, these results indicate that the new formulations have the capability of being substituted for the old standard to-be-discontinued insulations.

In addition, a comparison was made between DL1552A and RDL5654 in a low mach test char motor using TP-H1148 propellant and a 30.8 second firing time at an average pressure of 881 psi. The following results indicate that RDL5654 is a viable replacement for DL1552A. TABLE 10A COMPARISON OF THE MATERIAL AFFECTED RATE (MAR) OF DL1552A AND RDL5654 IN THE LOW MACH CHAR MOTOR D-5 DL1552A RDL5654 Mach No. Average MAR Average MAR Range (mils/sec.) (mils/sec.) Test motor HPCAT-07 MIR-05 Low velocity 0.0027 3.83 3.86 test section Medium velocity 0.0027-0.0081 3.79 3.80 test section High velocity 0.0094-0.044  5.36 5.49 test section

In the following Table 11 there are set forth examples of the new EPDM insulation formations, which now contain carbon fibers.

Batches of carbon fiber-containing insulations generally may be formulated in a mixer via a two-pass mix procedure. In the first pass, the mixer speed may be set to approximately 40 rpm and all of the components other than the curing agents, accelerators, and carbon fiber are added to the internal mixer. As the mixing proceeds, the temperature will increase up to about 300° F., or even higher. Suitable mixing times depend on the temperature and mixing speed, and are ascertainable to the skilled artisan without an undue amount of experimentation.

In the second pass, the mixer speed may be set to about 40 rpm, but this speed is dependent on the size of the mixer. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added to the mixer, then the remaining half of the master batch was added to the mixer. The second mixing step may be performed at a temperature rising to approximately 180° F. to 190° F. but below the temperature where the accelerator would become unduly activated. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature. Then the material is dissolved in a suitable solvent, such as, by way of example, hydrocarbons such as hexane, heptane, and/or cyclohexane. The carbon fiber was then mixed with, for example, a sigma-blade mixer. The material is then sheeted out and the solvent allowed to evaporate at ambient atmosphere or in an oven. Throughout this process, care must be taken that the frangible carbon fibers are not themselves fractured and broken up so as to become shortened and less effective as a result.

The shaping and curing of the inventive insulation may be performed in accordance with techniques known in the art.

The following Table 11 presents a number of examples of the novel EPDM formulations with carbon fibers. TABLE 11 EXAMPLES OF NEW EPDM FORMULATIONS WITH CARBON FIBER Ingredient RDL5421 RDL5420A RDL5377A RDL5444A RDL5443 RDL5436 RDL5408 RDL5409 KELTAN ® 4506 KELTAN ® 1446A KELTAN ® 2308 NORDEL ® IP 4520 40.0 40.0 45.0 45.0 45.0 80.0 80.0 NORDEL ® IP 4640 50.0 50.0 45.0 45.0 45.0 80.0 HYPALON ® 20 10.0 10.0 10.0 10.0 10.0 10.0 NEOPRENE ® FB NEOPRENE ® TW NEOPRENE ® GRT 20.0 20.0 TRILENE ® 67A 10.0 AKROCHEM ® P-133 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 AGERITE ® Resin D 2.0 2.0 2.0 AGERITE ® Stalite S 2.0 2.0 2.0 2.0 2.0 HISIL ® 233 3 3 3 3 3 3 3 3 VMC Carbon Fiber All formulations adjusted to have 26.75% VMC Carbon Fiber in finished product C. B. N330 1 1 1 1 1 1 1 1 ELASTOMAG ® 170 0.50 0.50 Zinc Oxide 4.0 5.0 5.0 5.0 4.0 4.0 4.00 4.00 Butyl Zimate 0.40 0.40 0.50 RHENOCURE ® AP-5 3.15 3.15 3.40 RHENOCURE ® AP-7 3.00 RHENOCURE ® AP-3 3.85 RHENOCURE ® ZADT/G 0.80 1.00 RHENOCURE ® S/G 0.80 ALTAX ® 1.00 1.00 1.10 AKROFORM ® Delta P.M. 0.35 SULFADS ® 0.50 Accelerator VS 2.70 2.70 AKROSPERSE ® IS-70 1.00 1.00 1.00 1.00 1.00 Sulfur 0.80 0.40 0.50 Ingredient RDL5445 RDL5410 RDL5375 RDL5395 RDL5394 RDL5435 RDL5434 RDL5376A KELTAN ® 4506 70.0 73.0 55.0 55.0 KELTAN ® 1446A 20.0 17.0 KELTAN ® 2308 35.0 35.0 NORDEL ® IP 4520 80.0 80.0 NORDEL ® IP 4640 80.0 80.0 HYPALON ® 20 10.0 10.0 10.0 10.0 10.0 NEOPRENE ® FB 20.0 20.0 NEOPRENE ® TW 20.0 NEOPRENE ® GRT TRILENE ® 67A 10.0 AKROCHEM ® P-133 5.0 5.0 5.0 5.0 5.0 5.0 AGERITE ® Resin D 2.0 2.0 2.0 2.0 AGERITE ® Stalite S 2.0 2.0 2.0 2.0 HISIL ® 233 3 3 3 3 3 3 3 3 VMC Carbon Fiber All formulations adjusted to have 26.75% VMC Carbon Fiber in finished product C. B. N330 1 1 1 1 1 1 1 1 ELASTOMAG ® 170 0.5 Zinc Oxide 4.0 4.0 4.0 4.0 4.0 4.0 4.0 5.0 Butyl Zimate 0.15 0.50 0.50 RHENOCURE ® AP-5 4.45 3.50 3.50 RHENOCURE ® AP-7 RHENOCURE ® AP-3 2.90 RHENOCURE ® ZADT/G .80 RHENOCURE ® S/G 0.60 ALTAX ® 1.10 1.10 1.20 1.10 AKROFORM ® Delta P.M. 0.45 0.20 SULFADS ® 0.50 Accelerator VS 2.70 2.70 AKROSPERSE ® IS-70 1.34 1.10 1.20 1.20 1.40 1.00 Sulfur 0.35 0.45

Silica Powder-Filled Insulation Properties

Table 12 reports rheological properties measured for the silica powder thermal insulation formulations that were set forth in Table 6. TABLE 12 RHEOLOGICAL PROPERTIES (silica powder) Mooney Oscillating disk rheometer (ODR at viscosity 320° F., 5° arc) (ASTM D 2084) (ML 1 + 4 ML MH ts2 (time to a Tc Silica-Filled at 212° F.) (minimum (maximum 2 point rise (90% EPDM (ASTM D torque, torque, at 1 above ML, cure) Formulation 1646) in.-lb.) hr., in.-lb.) in min.) (min) RDL 5338 72.0 17.7 93.9 6.0 43.5 RDL 5342 78.5 21.3 97.8 7.0 45.0 RDL 5343 78.8 19.9 98.0 4.3 44.5 RDL 5347 84.0 21.7 94.5 3.4 38.0 RDL 5363 83.0 22.9 87.2 3.3 44.2 RDL 5255 85.7 19.4 103.2 3.6 28.5 RDL 5335 87.7 23.4 93.4 2.7 38.5 RDL 5201 73.0 16.4 89.0 6.2 31.5 RDL 5220 71.2 16.0 110.0 5.9 40.0 RDL 5370 80.0 20.3 104.6 5.6 32.0 RDL 5380 80.3 21.0 107.4 7.3 56.0 RDL 5381 79.6 21.0 104.0 4.8 36.0 RDL 5194 62.0 13.4 90.9 8.7 44.0 RDL 5222 61.9 13.5 92.8 6.3 45.6 RDL 5273A 64.5 14.7 97.1 3.8 42.0 RDL 5298 51.0 11.0 91.0 7.1 49.0 RDL 5277 72.4 18.9 98.5 1.9 42.6 RDL 5279 75.0 17.3 93.0 2.7 18.8 RDL 5319 87.8 19.9 97.0 6.3 43.0 RDL 5320A 86.5 20.7 103.0 4.0 41.6 RDL 5350 83.0 22.0 103.0 6.8 46.1 RDL 5351A 84.6 19.8 105.6 3.5 37.0 RDL 5367 86.0 22.3 96.7 3.2 43.3 RDL 5185 79.5 18.7 90.0 8.4 40.9 RDL 5186 80.6 19.3 95.5 4.1 43.0 RDL 5205 79.9 19.4 94.0 8.0 41.2 RDL 5276 81.6 18.5 96.7 6.2 42.5

Table 13 reports various physical properties measured for the silica powder thermal insulation formulations that were set forth in Table 6. TABLE 13 PHYSICAL PROPERTIES Silica-Filled EPDM Formulation A B C D E F G RDL 5338 2280 646 2260 658 220 62.0 1.06 RDL 5342 1910 588 1790 582 206 63.4 1.06 RDL 5343 2330 655 2040 619 221 63.2 1.06 RDL 5347 2100 621 2060 630 271 68.6 1.06 RDL 5363 2920 610 2820 608 270 72.0 1.06 RDL 5255 2430 703 2330 695 223 69.4 1.10 RDL 5335 2560 725 2600 739 232 65.2 1.10 RDL 5201 2150 772 2220 779 190 70.2 1.10 RDL 5220 2170 634 2020 620 194 70.2 1.10 RDL 5370 2550 700 2340 688 201 70.4 1.10 RDL 5380 2610 688 2300 663 206 69.4 1.11 RDL 5381 2420 700 2470 701 207 69.0 1.11 RDL 5194 1920 734 1920 748 201 71.8 1.05 RDL 5222 2100 703 1960 653 218 71.2 1.05 RDL 5273A 1420 612 1430 613 241 70.4 1.06 RDL 5298 1910 592 1870 586 223 71.6 1.06 RDL 5277 2010 614 2190 632 181 66.4 1.05 RDL 5279 2390 672 2370 687 199 64.8 1.05 RDL 5319 2440 659 2270 647 205 71.2 1.05 RDL 5320A 2250 657 2220 653 224 71.6 1.06 RDL 5350 2350 688 2420 707 233 64.2 1.06 RDL 5351A 2270 681 2250 699 226 65.2 1.06 RDL 5367 2440 621 2430 625 238 70.4 1.06 RDL 5185 2310 776 2230 771 208 71.4 1.06 RDL 5186 2390 725 2290 718 221 71.6 1.06 RDL 5205 1990 729 2120 773 207 71.6 1.06 RDL 5276 2270 653 2210 656 223 71.2 1.06 Key: A = Tensile strength, parallel (psi) (ASTM D 412) B = Elongation Parallel (%) (ASTM D 412) C = Tensile Strength Perpendicular (ASTM D 412) D = Elongation Perpendicular (ASTM D 412) E = Tear Strength (ASTM D 412) F = Shore A hardness (ASTM D 2240) G = Specific gravity (ASTM D 792)

Carbon-Fiber Containing Insulation Formulation Properties

Next, Table 14 lists the rheological properties measured according to the indicated tests for the carbon fiber-containing insulation formulations set forth in Table 11. TABLE 14 RHEOLOGICAL PROPERTIES Mooney Oscillating Disk Rheometer (ODR at viscosity 300° F., 5° arc) (ASTM D 2084) (ML 1 + 4 ML MH ts2 (time to a Tc Carbon-Fiber at 212° F.) (minimum (maximum 2 point rise (90% EPDM (ASTM D torque, torque, at 1 above ML, cure) Formulation 1646) in.-lb.) hr., in.-lb.) in min.) (min) RDL 5421 72.4 23.3 111 3.5 30.2 RDL 5420A 70.0 21.2 119 2.9 37.0 RDL 5377A 68.0 18.8 102 2.8 36.5 RDL 5444A 67.4 21.0 110 3.2 28.0 RDL 5443 74.5 25.7 101 1.6 33.5 RDL 5436 80.0 25.8 98.2 2.6 32.0 RDL 5408 83.0 29.2 95.5 2.5 16.5 RDL 5409 83.8 25.8 99.0 3.5 25.5 RDL 5445 56.0 15.8 82.0 3.4 39.2 RDL 5410 81.0 18.0 103 4.8 34.5 RDL 5375 55.0 21.0 100 2.5 31.7 RDL 5395 64.7 29.2 87.4 3.2 35.0 RDL 5394 67.0 25.1 89.5 1.8 36.0 RDL 5410 81.0 18.0 103 4.8 34.5 RDL 5434 43.7 12.1 136 1.9 18.8 RDL 5435 39.0 9.1 118 2.3 19.0

Table 15 reports the physical properties measured for the carbon fiber-containing insulation formulations set forth in Table 11. TABLE 15 PHYSICAL PROPERTIES Carbon-Fiber EPDM Form. A B C D E F RDL 5421 1750 3.34 820 4.59 84.4 1.097 RDL 5420A 1600 3.12 871 4.60 84.2 1.106 RDL 5377A 1540 3.11 814 4.03 82.6 1.072 RDL 5444A 2030 5.01 884 6.30 84.2 1.097 RDL 5443 2070 5.37 799 7.21 83.2 1.090 RDL 5436 1500 2.48 721 4.05 84.0 1.098 RDL 5408 1060 3.77 525 5.61 81.2 1.131 RDL 5409 1112 2.95 668 3.35 83.4 1.142 RDL 5445 1160 3.58 725 4.71 84.4 1.096 RDL 5410 1500 2.88 719 3.85 83.8 1.063 RDL 5375 2010 4.38 881 6.85 80.6 1.076 RDL 5395 2650 4.55 1090 4.98 86.1 1.078 RDL 5394 2218 3.80 982 4.84 87.2 1.088 RDL 5435 1690 4.17 905 6.03 83.4 1.127 RDL 5434 1470 4.41 786 5.82 83.6 1.126 Key: A = Tensile strength, parallel (psi) (ASTM D 412) B = Elongation Parallel (%) (ASTM D 412) C = Tensile Strength Perpendicular (ASTM D 412) D = Elongation Perpendicular (ASTM D 412) E = Shore A hardness (ASTM D 2240) F = Specific gravity (ASTM D792)

The following Table 16 shows the results of ablation tests with the silica powder-containing formulations of Tables 6, 12 and 13. TABLE 16 MATERIAL ABLATION RATE (MILS/SECOND) Silica powder Low Velocity, Filled 0.003 Mach RDL 5338 3.65 RDL 5343 3.14 RDL 5347 2.31 RDL 5363 2.82 RDL 5255 2.76 RDL 5335 2.42 RDL 5220 3.66 RDL 5370 3.44 RDL 5380 3.24 RDL 5298 3.36 RDL 5319 3.23 RDL 5320 2.81 RDL 5350 3.38 RDL 5351 3.35 RDL 5367 3.31 RDL 5186 2.34

The ablation tests were performed as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the three velocity regions of the motor (low, medium, and high Mach) were neat molded using conventional compression molding techniques. The low Mach region had space for 15 specimens, while the medium and high Mach regions had space for 4 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 12.1 seconds and 860 psi, respectively.

The following Table 17 shows the results of ablation tests with the carbon fiber-containing formulations of Tables 11, 14 and 15. TABLE 17 MATERIAL ABLATION RATE (MILS/SECOND) Carbon fiber Low Velocity, Medium Velocity, High Velocity, Avg. EPDM 0.003 Mach Avg. 0.03-0.09 Mach 0.01-0.15 Mach RDL 5421 3.38 11.4 18.7 RDL 5420 2.87 12.2 19.2 RDL 5377 3.86 11.5 17.6 RDL 5444 3.29 10.7 18.4 RDL 5443 3.22 12.5 19.9 RDL 5436 2.63 11.0 15.3 RDL 5408 3.91 11.2 16.5 RDL 5409 4.12 10.6 15.5 RDL 5410 3.55 10.9 19.9 RDL 5375 2.97 12.3 16.0 RDL 5395 3.00 11.7 20.8 RDL 5394 3.05 11.7 20.2 RDL 5435 3.90 11.4 15.1 RDL 5434 3.90 11.3 15.3 RDL 5376 4.27 12.9 14.7 RDL 5445 3.45 11.3 18.3

The ablation tests were also performed as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the low-velocity region of the motor were molded using conventional compression molding techniques. The low Mach region had space for 15 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 12.1 seconds and 860 psi, respectively.

From the characteristics measured in these tables, it can be seen that a special combination of properties is required for a suitable insulation. It is important to the consideration of this invention that the formulations are effective as to both of the commonly used solid additives, silica powder and carbon fibers.

With reference to the results set forth in the foregoing tables, it is presently considered in Table 13 (silica-filled EPDM) that the minimum acceptable values for tensile strength (parallel and perpendicular, values A and C) should be at least about 1600, and are preferably close to equal (within about plus or minus 10%). The elongation (parallel and perpendicular, Table 13) should be in the range of about 550-850. For tear strength, the value of about 170 is presently seen as a minimum characteristic. Similarly, an ODR-measured maximum torque MH (Table 12) for the silica powder-filled insulations should be at least about 85 in.-lb. and at most about 120 in.-lb., preferably, at most about 115 in.-lb. is currently thought to be appropriate. The ODR torque ML has a useful lower limit of about 5 in.-lb., preferably about 10 in.-lb. The scorch test Ts result (Tables 12 and 14) should be at least 1.5. For silica-filled insulations, the tear strength (Table 13) should be at least about 170. For carbon fiber-containing insulations, the elongation should exhibit a minimum of at least about 2%, preferably at least about 2.5%, especially for measurement B in Table 15. Also, for the carbon fiber insulations (Table 14) the Mooney viscosity ML should be below about 90 Mooney units.

Considering the combined properties of the individual insulations noted in the above tables, the silica-filled insulation 5273A is not presently preferred nor are the carbon fiber-containing insulations 5408, 5409, 5434 and 5445.

In addition, formulations were also made using aramid fibers as the filler material. An exemplary EPDM terpolymer for use with KEVLAR® fibers and comprising ENB as its diene component is NORDEL® IP NDR-4640 (available from DuPont Dow Elastomers), which may be substituted into the R196 for the NORDEL® 1040 without requiring significant modifications to the R196 formulation. KEVLAR® fibers are formed from fibers of poly(p-phenyleneterephathalamide). Generally, the one significant modification that is required with the use of aramid fibers involves a reduction in the amount of the curing agent, e.g., peroxide, again to offset the increased reactivity (relative to HD) of the alkylidene norbornenes. Furthermore, NORDEL® IP NDR-4640 is not presently foreseen as being susceptible to obsolescence issues.

Thus, in accordance with a further embodiment of this invention utilizing aramid fibers, the inventive rocket motor insulation formulation comprises, as a polymeric base, an EPDM/polyisoprene blend in which the diene component of the EPDM is based on at least one alkylidene norbornene, such as ethylidene norbornene (ENB), in effect in place of and without the above-described primary EPDM. The alkylidene norbornene content of such an EPDM may be from about 2 wt %-10 wt %. The selected EPDM terpolymer should be substitutable into the R196 thermal insulation formulation (Table 4) without requiring significant modification of the formulation. In accordance with a preferred embodiment, the EPDM terpolymer is NORDEL® IP NDR-4640, which includes ENB as its diene monomer, is available from DuPont Dow Elastomers, and is manufactured in a relatively large capacity.

An exemplary aramid fiber formulation is set forth in Table 18: TABLE 18 Ingredient Parts by Weight EPDM terpolymer with ENB 79.5-80.5 diene monomer component Polyisoprene 19.5-20.5 Antioxidant 0.95-1.05 Halogen-containing flame 39.5-40.5 retardant Metal oxide flame retardant 19.5-20.5 KEVLAR ® fiber (aramid 19.5-20.5 staple fiber) Curing agent 1.45-1.55

In a more preferred embodiment, the polyisoprene, antioxidant, flame retardants, and aramid, e.g., KEVLAR® fibers are the same as those set forth in Table 4 above and are present in the concentrations specified in Table 4. The metal oxide may be present in fine powdery form so as to also serve as a filler. VAROX® DBPH-50 is also a suitable peroxy curing agent, although the substitution of ethylidene norbornene for 1,4-hexadiene requires a reduction in curing agent concentration to offset the increased reactivity of ethylidene norbornene. By providing the reformulated thermal insulation formulation with a peroxide concentration in a range of from about 1 to about 2.5 phr, more preferably about 1.5 phr, a curing effect is achieved similar to the properties of R196.

In general, the EPDM/polyisoprene combination may be used with about 60 wt %-90 wt % of the norbornene EPDM and about 40 wt %-10 wt % polyisoprene, again with suitable additives being present in functionally desired amounts.

Suitable additives that may be added as functionally required or as desired include one or more of the following, in various combinations: fillers, antidegradants, curing agents, plasticizers, processing aids, pigments, bonding agents, fibers, and flame retardants. Two classes of curing systems that may be used are sulfur-based curing agents in combination with organic accelerators and peroxide curing agents.

The shaping and curing of such aramid inventive insulations may be performed in accordance with techniques known in the art.

EXAMPLES OF ARAMID FIBER INSULATIONS

Individual batches of insulation formulations were prepared USING NORDEL® IP NDR-4640 (Example, in Table 20) and NORDEL® IP NDR-1040 (Comparative Example B). TABLE 19 Ethylene Propylene Diene Polymer content content content/type Viscosity NORDEL ® 55 40 5/ENB 40 IP NDR-4640 NORDEL ® 1040 55 41 4/HD 40

The batches were each formulated in a laboratory scale Reliable Rubber & Plastics Machinery Company Model R-260 internal mixer having a net chamber volume of 4260 cubic centimeters. A 3000 gram batch was prepared by a two-pass mix procedure.

In the first pass, the mixer speed was set at 40 rpm, and 80 parts by weight of the corresponding EPDM component set forth in Table 3, 20 parts by weight of NATSYN® 2200 polyisoprene, and 1.0 part by weight of Wingstay S were added to the mixing chamber and masticated for one minute. Subsequently, 40 parts by weight of Dechlorane Plus 515, 20 parts by weight of Sb₂O₃, and 20 parts by weight of ¼ inch KEVLAR® aramid fibers were added, and the mixer speed was increased to 60 rpm. The batches were removed from the internal mixer after mixing for a total of approximately 7 minutes. The temperature at the time of removing the batch was 300° F. to 320° F. The master batch was sheeted out on a 6″×13″ Farrel Corporation laboratory two-roll mill and allowed to cool to room temperature.

For the Example and Comparative Example B, the fibers were dispersed in a two-roll mill to reduce agglomerations of fibers. The dispersion was accomplished by tightening the nip (separation between the two rolls) to 0.030 inch on the laboratory two-roll mill and passing the rubber through the nip a minimum of six times. This dispersion step was performed after the first pass but before the second pass. The rubber was then allowed to cool before the second pass.

In the second pass, the mixer speed was set to 40 rpm. In a sequential manner, half of the master batch was added to the mixer, then the curatives were added at a peroxide level of 1.5 phr, then the remaining half of the master batch was added to the mixer. After a total mix time of approximately 40 seconds, the final mix was removed from the mixer. The actual temperature at the time of removal was approximately 200° F. The material was sheeted out onto the laboratory mill and allowed to cool to room temperature prior to evaluating its rheological and physical properties.

Table 20 gives the measured rheological properties of these examples. TABLE 20 RHEOLOGICAL PROPERTIES Comparative Property Example Example B (1) Mooney viscosity (ML 1 + 4 at 212° F.) 58.2 51.3 (ASTM D 1646) (2) Mooney scorch (MS +1 at 270° F., 52.0 27.8 min.) (ASTM D 1646) (3) Oscillating disk rheometer (ODR at 320° F., 5° arc) (ASTM D 2084) properties: (a) ML (minimum torque, in.-lb.) 11.3 13.3 (ASTM D 2084) (b) MH (maximum torque, at 2 hrs, in.-lb.) 61.5 55.0 (ASTM D 2084) (c) Ts2 (time to 2 point rise above 4.3 3.3 minimum viscosity, in min.) (ASTM D 2084) (d) MC(90) (in.-lb.) (ASTM D 2084) 56.4 50.8 (e) Tc (90) (min) (ASTM E 2084) 65.5 63.0

Table 21 reports the physical properties of these aramid fiber examples. TABLE 21 PHYSICAL PROPERTIES Specification Comparative Properties Requirement Example Example B Density (lbs/in³) (ASTM D 792) 0.040 min. 0.0422 0.0421 Shore A hardness (ASTM D 70-85 75.8 74.4 2240) Tensile strength, parallel (psi)   700 min. 1085 1050 (ASTM D 412) Elongation parallel (%)   10 min. 27.3 24.6 (ASTM D 412) Tensile strength, perpendicular 461 367 (psi) (ASTM D 412) Elongation perpendicular (%) 109 126

Table 22 reports the results from a test to measure the ablation rates for these aramid fiber examples. TABLE 22 MATERIAL ABLATION RATE (MILS/SECOND) Comparative Example Example B Low Mach Region (0.0028) 3.58 3.43 Medium Mach Region (0.039-0.0970) 10.55 10.83 High Mach Region (0.109-0.140) 16.59 21.99

The ablation tests were performed as follows. The tests were performed in a char motor, schematically illustrated in FIG. 2. The samples for the three velocity regions of the motor (low, medium, and high Mach) were net molded using conventional compression molding techniques. The low Mach region had space for 15 specimens, while the medium and high Mach regions had space for 4 specimens. The thickness of each specimen was measured at several axial locations before and after firing. The char motor itself featured a propellant beaker filled with Thiokol Propulsion propellant TP-H1148 to a sufficient depth to provide for a 12-second burn with a tungsten nozzle radius of 0.24 inches. The actual action time and average operating pressure for the test motor were 12.0 seconds and 842 psi, respectively.

Although the Mooney scorch time of the Example was somewhat longer than that of the Comparative Example (R196), at the cure temperature of 320° F., the cure rates, as indicated by the 90 percent cure time tc(90), of the two materials were equivalent. Similarly, the physical properties of the materials prepared in accordance with the Example and Comparative Example B were substantially equivalent and well within the specification requirements. Additionally, the ablative performance of the material prepared in accordance with the inventive Example was equivalent or better than the ablative performance of the Comparative Example B (R196).

In the course of experimentation leading to this invention, a number of other candidate elastomeric formulations were tried in the hopes that the path to a new acceptable insulation would not be overly difficult. In fact, such other formulations proved to be unsuccessful and this is demonstrated by the following illustrative unsuccessful examples.

Rubber Component Formula RDL5191 Parts by Category Component Id. Weight Wt % Grams Polymers  1 NORDEL ® IP 80.00 54.22 1626.57   NDR-4640 Plasticizers  2 NEOPRENE ® FB 20.00 13.55 406.64 Antioxidants  3 AGERITE ® Resin D 2.00 1.36 40.66 Fillers  4 HISIL ® 233 35.50 24.06 721.79 Activator  5 Zinc Oxide 4.00 2.71 81.33  6 2^(nd) Pass Additions Accelerators  7 RHENOCURE ® S/G 0.80 0.54 16.27  8 RHENOCURE ® AP-3 3.85 2.61 78.28  9 RHENOCURE ® 1.00 0.68 20.33   ZADT/G Curative 10 Sulfur 0.40 0.27 8.13 Totals: 147.55 100.00 3000.00 Mixing Instructions, First Pass:

-   Add NORDEL® and zinc oxide -   Add HISIL® and antioxidant -   Add NEOPRERE® FB -   Dump about 280° F.     2nd Pass Additions -   Mix MB with curatives in Brabender -   Dump about 190° F. or below

Rubber Component Formula RDL5570 Parts by Category Component Weight Wt % Grams Polymers  1 NORDEL ® IP 50.00 33.89 1016.60   NDR-4640  2 NORDEL ® IP 40.00 27.11 813.28   NDR-4520 Plasticizers  3 HYPALON ® 20 10.00 6.78 203.32  4 AKROCHEM ® P-133 5.00 3.39 101.66 Antioxidants  5 AGERITE ® Stalite S 2.00 1.36 40.66 Fillers  6 HISIL ® 233 35.50 24.06 721.79 Activator  7 Zinc Oxide 4.00 2.71 81.33  8 2^(nd) Pass Additions Accelerators  9 Methyl Ethyl Tuads 2.00 1.36 40.66 10 Monex or Unads 1.00 0.68 20.33 11 SULFADS ® 1.00 0.68 20.33 12 Butyl Zimate 1.50 1.02 30.50 Curative 13 Sulfur 0.50 0.34 10.17 Totals: 152.50 103.35 3100.64 Mixing Instructions, First Pass:

-   Mix EPDMs and HYPALON® together -   Add antioxidants 5 and 6, add silica, zinc oxide -   Add AKROCHEM® resin -   Dump about 250-300° F.     2nd Pass Additions -   Mix MB with curatives in Brabender -   Dump about 190° F. or below

Rubber Component Formula RDL5571 Parts by Category Component Weight Wt % Grams Polymers  1 NORDEL ® IP 50.00 33.89 1016.60   NDR-4640  2 NORDEL ® IP 40.00 27.11 813.28   NDR-4520 Plasticizers  3 HYPALON ® 20 10.00 6.78 203.32  4 AKROCHEM ® P-133 5.00 3.39 101.66 Antioxidants  5 AGERITE ® Stalite S 2.00 1.36 40.66 Fillers  6 HISIL ® 233 35.50 24.06 721.79 Activator  7 Zinc Oxide 4.00 2.71 81.33  8 2^(nd) Pass Additions Accelerators  9 Methyl Tuads 0.75 0.51 15.25 10 ALTAX ® 1.50 1.02 30.50 11 SULFADS ® 0.75 0.51 15.25 12 Butyl Zimate 1.50 1.02 30.50 Curative 13 Sulfur 0.50 0.34 10.17 Totals: 151.50 102.68 3080.31 Mixing Instructions, First Pass:

-   Mix EPDMs and HYPALON® together -   Add antioxidants 5 and 6, add silica, zinc oxide -   Add AKROCHEM® resin -   Dump about 250-300° F.     2nd Pass Additions -   Mix MB with curatives in Brabender -   Dump about 190° F. or below

Rubber Component Formula RDL5572 Parts by Category Component Weight Wt % Grams Polymers  1 NORDEL ® IP NDR- 50.00 33.89 1016.60  4640  2 NORDEL ® 40.00 27.11 813.28  IP NDR-4520 Plasticizers  3 HYPALON ® 20 10.00 6.78 203.32  4 AKROCHIEM ® P-133 5.00 3.39 101.66 Antioxidants  5 AGERITE ® Stalite S 2.00 1.36 40.66 Fillers  6 HISIL ® 233 35.50 24.06 721.79 Activator  7 Zinc Oxide 4.00 2.71 81.33  8 2^(nd) Pass Additions Accelerators  9 Methyl Tuads 3.00 2.03 61.00 10 Methyl Zimate 3.00 2.03 61.00 11 Sulfasan R 2.00 1.36 40.66 12 Butyl Zimate 3.00 2.03 61.00 Curative 13 Sulfur 0.50 0.34 10.17 Totals: 158.00 107.08 3212.47 Mixing Instructions, First Pass:

-   Mix EPDMs and HYPALON® together -   Add antioxidants 5 and 6, add silica, zinc oxide -   Add AKROCHEM® resin -   Dump about 250-300° F.     2nd Pass Additions -   Mix MB with curatives in Brabender -   Dump about 190° F. or below

Rubber Component Formula RDL5172 Parts by Category Component Weight Wt % Grams Polymers 1 NORDEL ® IP NDR-4640 80.00 54.22 1626.57 2 TRILENE ® 67A 10.00 6.78 203.32 Plasticizers 3 HYPALON ® 20 10.00 6.78 203.32 4 AKROCHEM ® P-133 5.00 3.39 101.66 Antioxidants 5 AGERITE ® Resin D 2.00 1.36 40.66 Fillers 6 HISIL ® 233 35.50 24.06 721.79 Activator 7 Zinc Oxide 3.00 2.03 61.00 8 2^(nd) Pass Additions Accelerators 9 Captax 1.00 0.68 20.33 10 Tellurac 0.50 0.34 10.17 11 SULFADS ® 0.75 0.51 15.25 Curative 12 Sulfur 1.00 0.68 20.33 Totals: 148.75 100.81 3024.40 Mixing Instructions, First Pass:

-   Mix NORDEL® and HYPALON® together -   Add antioxidants 5 and 6, add silica, zinc oxide -   Add TRILENE® and AKROCHEM® resin -   Dump about 250-300° F.     2nd Pass Additions -   Mix MB with curatives in Brabender -   Dump about 190° F. or below

As indicated below, for various reasons, the above-described experimental test insulations proved to be unsuitable formulations for production of rocket motor insulations. TABLE 23 Unsuccessful Experimental Results Rheological Properties Formulation ML MH ts2 Tc(90) Rejected because: RDL5571 25.3 138.3 2.3 21.3 MH too high to match existing standard insulation RDL5572 22.6 173.9 2.5 25.7 MH too high to match existing standard insulation RDL5570 25.6 128.6 2.7 29.5 MH too high to match existing standard insulation RDL5191 26.8 133.6 3.5 25.4 MH too high to match existing standard insulation RDL5172 39.0 128.0 1.0 25.0 Uses the old cure system as in Table 2, but product is much too scorchy to be useful; cures at too low a temperature

As these unsuccessful formulations indicate, and faced with the discontinuance of the availability of the hitherto thought to be essential component for the present standard insulation formulations, the development of a suitable and acceptable solid propellant rocket motor insulation that will provide the required performance criteria has not been an easy task.

The foregoing detailed description of the preferred embodiments of the invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Many modifications and variations within the scope of this invention will be apparent to practitioners skilled in this art. The illustrated embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is, therefore, intended that the scope of the invention cover various modifications and equivalents included within the spirit and scope of the appended claims. 

1. An elastomeric insulation cured from an ethylene propylene diene monomer (EPDM) terpolymer composition, comprising: a primary EPDM terpolymer composition formulated from ethylene, propylene, and an alkylidene norbornene; polyisoprene; at least one filler selected from the group consisting of silica, carbon fibers, and aramid staple fibers; and a curing agent.
 2. The elastomeric insulation of claim 1, wherein the alkylidene norbornene is ethylidene norbornene.
 3. The elastomeric insulation of claim 1, wherein the alkylidene norbornene is present in the primary EPDM terpolymer composition in a range of between approximately 2 weight percent and approximately 12 weight percent.
 4. The elastomeric insulation of claim 1, wherein the ethylene is present in the primary EPDM terpolymer composition in a range of between approximately 40 weight percent and approximately 80 weight percent.
 5. The elastomeric insulation of claim 1, wherein the curing agent comprises a peroxide curing agent present in an amount of 1 phr to 2.5 phr.
 6. The elastomeric insulation of claim 1, wherein the curing agent comprises sulfur.
 7. The elastomeric insulation of claim 1, wherein the aramid staple fibers comprise poly(p-phenyleneterephthalamide).
 8. The elastomeric insulation of claim 1, further comprising a secondary polar polymer.
 9. The elastomeric insulation of claim 8, wherein the secondary polar polymer comprises a member selected from the group consisting of chlorosulfonated polyethylene and polychloroprene.
 10. The elastomeric insulation of claim 8, wherein the elastomeric insulation comprises from about 15 weight percent to about 25 weight percent of the secondary polar polymer.
 11. The elastomeric insulation of claim 1, further comprising at least one additive selected from the group consisting of an antioxidant, a plasticizer, a tackifier, a cure additive, and an adhesion promoter.
 12. The elastomeric insulation of claim 1, wherein the propylene content of the alkylidene norbornene ranges from approximately 22 weight percent to approximately 26.5 weight percent.
 13. The elastomeric insulation of claim 1, wherein the elastomeric insulation comprises from approximately 60 weight percent to approximately 90 weight percent of the primary EPDM terpolymer composition and from approximately 10 weight percent to approximately 40 weight percent of the polyisoprene. 